1. Field of the Invention
The present invention relates to a method for making a nano-scale grain metal powders and a method of making a high-frequency soft magnetic core using the same, and more particularly, to a method for making magnetic powders obtained by thermal treatment crystallizing and then crushing amorphous ribbons produced using a rapid solidification process (RSP), and a method for making a high-frequency soft magnetic core by using the same.
2. Description of the Related Art
In general, a Fe-based amorphous soft magnetic body which is used as a conventional high-frequency soft magnetic body has a high saturation magnetic flux density (Bs), but has a low magnetic permeability, a large magnetic deformation, and an inferior high-frequency characteristic. A Co-based amorphous soft magnetic body has a low saturation magnetic flux density and a drawback of an expensive raw material. In case of an amorphous soft magnetic alloy, it is difficult to shape it in the form of a strip, and is limited to form a product of a toroidal shape. Since a ferrite soft magnetic body has a low high-frequency loss and a small saturation magnetic flux density , it is difficult to accomplish a compact product. Both of the amorphous and ferrite soft magnetic body has bad reliability in thermal stability due to a low crystallization temperature.
An amorphous ribbon fabricated by a rapid solidification process (RSP) is wound to then be used as a soft magnetic core. In this case, the soft magnetic core has a remarkably low direct-current overlapping characteristic and a remarkably low high-frequency characteristic, as well as an inferior core loss. This is because a powder core product has an effect of uniformly distributing an air gap by forming an insulating layer between powder particles, but has no air gaps in the case of an amorphous ribbon wound core. Thus, a core which is formed by using an amorphous ribbon in order to improve a direct-current overlapping characteristic, has a thin gap. In this case, an efficiency is lowered and an electromagnetic wave can be influenced to other electronic products and the human body, due to a leakage flux produced from the gap.
Soft magnetic cores which are used in choke coils for suppressing or smoothing electronic noise are manufactured in a manner that ceramic insulation materials are coated on magnetic metal powder such as pure iron, Fe—Si—Al alloy (referred to as “Sendust” hereinbelow), Ni—Fe—Mo Permally (referred to as “MPP (Moly Permally Powder)” hereinbelow), and Ni—Fe Permally (referred to as “high flux” hereinbelow), and then forming lubricants are added on the coated metal powder, to then be formed by pressure and thermally treated.
First, a pure iron powder core has an advantage that a price of the core is inexpensive. However, a core loss is relatively very large. Accordingly, when the core is overheated during operation and high direct-current (DC) current is overlapped, a magnetic permeability is greatly lowered.
Meanwhile, an MPP core has an excellent frequency characteristic at a frequency ranging from 100 kHz to 1 MHz, and has the lowest core loss among all kinds of the metal powder. Also, the MPP core has an advantage that a magnetic permeability is a little reduced even with an high direct-current (DC) overlap. However, since the MPP core is very expensive, it is difficult to employ it in view of production cost.
A high flux core has an excellent frequency characteristic at a frequency ranging from 100 kHz to 1 MHz, and has the low core loss. Also, the high flux core has an advantage that a magnetic permeability is reduced in the least even with an high direct-current (DC) overlap, among all metal powder cores.
Also, a “Sendust” core has a very low core loss value in comparison with a pure iron core. The “Sendust” core has a frequency characteristic equivalent to those of the MPP and high flux cores, and is cheaper approximately half than the MPP and high flux cores. However, a direct-current (DC) overlapping characteristic at a flow of large current is relatively lower than those of the MPP and high flux cores. As a result, the “Sendust” core is limited in use at adverse circumstances.
A ferrite soft magnetic material has an advantage of a good magnetic permeability or a small core loss at a frequency of 500 kHz or higher, but has a small saturation magnetic flux density. As a result, the ferrite soft magnetic material is limited in making a compact and light product.
Thus, a variety of metal powder is employed in making a smoothing choke coil in a SMPS (Switching Mode Power Supply) in various forms for each use, considering a price, a core loss, a direct-current overlapping characteristic, and a core size. However, all kinds of the conventional metal powder cores can be used only at a frequency of 1 MHz or lower, but is limited in using it at a frequency of 1 MHz or higher.
Here, a direct-current overlapping characteristic is a magnetic core characteristic with respect to a waveform formed by feeble alternating-current, generated during converting an alternating-current input of a power supply into a direct-current, on which direct-current is overlapped. In the case that direct-current is overlapped over alternating-current, a core magnetic permeability is lowered in proportion with the direct-current. Here, the direct-current overlapping characteristic is estimated in a ratio (% μ-percent permeability) represented as a direct-current overlapping permeability with respect to a permeability at the state where direct-current is not overlapped,
Meanwhile, in the conventional art, an insulation layer is formed between powder particles during making a soft magnetic core, to thereby uniformly distribute an air gap. Accordingly, an Eddy current loss sharply increasing at high-frequency is minimized, and the air gap is maintained in whole, to thereby accomplish an excellent direct-current overlapping characteristic at large current. However, the conventional are has a problem of lowering a magnetic permeability at a frequency band of 1 MHz or higher.